1. Technical Field
This disclosure relates to a display device having a backlight unit that illuminates a display panel from the rear face, and a method for controlling the display device.
2. Description of the Related Art
A display device, having a display panel that uses non-self-light-emitting liquid crystal as a light modulation element, includes a backlight unit that illuminates the display panel from the rear face, and displays arbitrary images by controlling the transmittance of the light emitted from the backlight unit using the liquid crystal. For the light source of the backlight unit, light emitting diodes, for example, are used, hence various techniques to control driving of the light emitting diodes have been proposed (e.g. see Japanese Patent Application Laid-Open No. 2008-91311).
In the case of the light emitting diode drive device according to Japanese Patent Application Laid-Open No. 2008-91311, a plurality of light emitting diodes are connected in parallel, and when pulsed current is applied to each light emitting diode sequentially at a predetermined interval with a same pulse width, a cycle of applying pulsed current is sequentially shifted by a predetermined cycle in each light emitting diode connected in parallel. If all the light emitting diodes were driven at a same timing, pulsed driving control with large current as a whole would be performed, which would cause noise to occur. However, the above-described control can reduce noise to occur.
In this technique disclosed in Japanese Patent Application Laid-Open No. 2008-91311 however, light emitting diodes are turned ON with a predetermined brightness during one frame period, and there is an overlapped period in which the respective periods of being turned on overlap among the light emitting diodes connected in parallel. Therefore, the current capacity required for driving the light emitting diodes increases by an amount equal to the number of light emitting diodes that simultaneously turn ON in the overlapped period, compared with the case where there is no overlapped period. Accordingly, the cost required for driving the light emitting diodes increases.
In a display device, in which the display unit is segmented into a plurality of areas, including a backlight unit which has a plurality of light source units for illuminating each of the plurality of areas, using a black insertion technique is known, in which a period to display a black image by turning all the light source units off is provided in one frame period, or a period to display a black image in an area of a part of the display unit is provided by shifting the phase to turn each of the light source units on, respectively. With this technique, display of the display device using hold type liquid crystal can be closer to the impulse type display, so as to improve the resolution of video images. In a display device using this technique as well, the brightness in one frame period must be kept at a predetermined level or more. Further, as the power supply unit to drive this light source unit, a power supply unit having a current capacity matching with the overlapped period, in which a plurality of light source units are turned on simultaneously, must be provided. Recently, the current capacity required for the overlapped period, where a plurality of light source units are turned on simultaneously, is increasing, since high brightness in a short light on time is demanded for each light source unit. Therefore, the load on the light source units is becoming excessive.